1. Field of the Invention
The present invention relates generally to a control device for a camera shutter and a control method therefor and, more particularly, to a control device and method for controlling the opening time of a shutter for a camera in an aperture-priority mode.
2. Description of Related Art
An exposure value (Ev) of a camera is a function of a diaphragm value of the shutter and the opening time therefor. Therefore, the opening time of the shutter is determined by setting a predetermined exposure value and the diaphragm value.
An operation of driving a shutter is schematically shown in FIG. 5 wherein the abscissa and ordinate represent the time (t) and opening size (Av), respectively. In FIG. 5, the shutter starts its opening motion at a point A, is fully opened at a point B, starts its closing motion at a point C after an opening time (t) elapses, and then is completely closed at a point D. The resulting waveform comprises inclined linear segments AB and CD and a horizontal linear segment BC to define, with the abscissa, a trapezoid configuration. The area of the trapezoid which is enclosed by the points A, B, C and D corresponds to the exposure value (Ev) for this shutter driving operation. The shutter driving operation which provides such a trapezoidal waveform for the opening and closing operations of the shutter is referred to as a "trapezoidal control". Thus, when the diaphragm value is changed, a time period for holding a predetermined opening size must be changed to obtain the same exposure value. That is, when the opening size is changed from a certain value (Av) to a value (Av') smaller than the value (Av), the opening period of time is changed to AD' and an area of a trapezoid surrounded by AB'C'D'A becomes the corresponding exposure value.
In the above-described exposure operation, the opening and closing operations of the shutter denoted by the inclined linear segments AB and CD in FIG. 5 are controlled by a stepping motor in a stepwise manner. Since a time difference exists between the rotation of the stepping motor and the operation of shutter vanes, the above-described waveform becomes a distorted trapezoid.
In a conventional method for controlling the opening time of the shutter in which the exposure amount is controlled by a stepping motor, etc., the relationship between the exposure value (Ev) and the opening time (t) at each of predetermined diaphragm values is stored beforehand as a data table in a storing unit of a control device, and the opening time for a desired shutter driving operation is determined on the basis of the data table.
When the trapezoidal control is carried out in the aperture-priority mode, the opening time is varied in accordance with the diaphragm value. For example, if the diaphragm value is set at a value smaller than the value for the full-opened diaphragm, the opening time must be set to be longer to obtain the same exposure value as set for the diaphragm full-opening operation. Therefore, in order to control the opening time with the data table, a data table must be individually prepared for each and every exposure value in order to cover all diaphragm values, and thus the number of the data tables is greatly increased.
In Japanese Unexamined Patent Publication No. JP-A-4-319924, applicants have proposed the following in an effort to resolve the above-described drawbacks in the conventional art. That is, in controlling the opening and closing operations of a shutter and the opening time based on the set exposure value of a camera, the set diaphragm values of a shutter and the like, the opening time for a set diaphragm value is calculated and determined based on the opening size at a predetermined diaphragm value of the shutter, and thus the amount of data stored in the data tables is reduced.
Although the capacity of the storing unit of the control device could be reduced by the above proposal, the opening time of the shutter and the like in accordance with the opening size at each of various diaphragm values, the time periods required for the shutter opening and closing operations, and the exposure value in accordance with the brightness of an object or the like are also required to be stored in the storing unit, and calculations are carried out based on such data. Furthermore, data pertaining to the exposure value must be stored in fine steps in order to perform a highly fine exposure value control. Thus the amount of data required to be stored in the storing unit of the control device is greatly increased, and the points of adjustment in the control device become numerous, thereby requiring a long period of time for processing the adjustment and for conducting the overall exposure operation.